Standard PET bottle polymer typically has an intrinsic viscosity, or IV in the range of 0.76 to 0.84 dl/g. Copolymer modification (acid or glycol) has been used to decrease the crystallization rate and widen the injection molding processing window. Standard PET bottle polymers with copolymer modification typically have higher IV in order to compensate for the increase in natural stretch ratio associated with the copolymer modification; and to compensate for the expected drop in bottle physical properties such as topload, structural strength, creep resistance, etc associated with copolymer modification.
Processing equipment and technology have been developed to reduce bottle weight by increasing the preform to bottle stretch ratio, or highly stretching the preforms. In standard bottle designs, higher IV has been associated with improved bottle physical properties because of the generally better orientation at standard planar stretch ratios. It seemed natural that a higher IV would be desirable in the lighter bottles using higher planar stretch ratios, and that increasing polymer IV would result in better physical properties. Thus, container molders have specified that PET with IV values between about 0.80 to about 0.84 dl/g be used for the highly stretched, lighter weight bottles. Since bottle weight reduction was the result of increasing the planar stretch ratio from the standard 11-13 to approximately 14-15.5, and the standard PET bottle polymer had been designed to process at the lower stretch ratios; many problems were encountered with the new designs. This has resulted in extremely difficult, often impossible processing because the preforms made from PET polymer with the recommended IV could not stretch to the designed planar stretch ratio and achieve proper material distribution and clarity; regardless of the processing changes made. In other cases; the processing window using the new designs was prohibitively narrow; and bottle properties were not consistent.